Fusion welding sockets are used generally in joining together plastic pipes and in connecting plastic pipes to pipe bends, T-pieces and the like. The welding sockets are usually provided with inwardly projecting shoulders in their center regions and incorporate a resistance coil on each side of the shoulders. When joining two pipes together, the pipe ends can be inserted into the socket until stopped by the shoulders. The person coupling the pipes will then know that the pipes have been centered correctly in the socket. A voltage source is then connected to the coils. The material in the inner surface of the socket and in contact with the material at the pipe ends will therewith fuse together locally as a result of the heat generated in the resistance wires.
This represents a very simple and effective method of joining together simple pipe conduits. Many installations, for instance gasoline transporting installations, require the use of double walled conduits, i.e. conduits which include two coaxial pipes and which are constructed so as to provide an annular space between the radially inner and the radially outer pipe. Such an annular space of a double walled conduit is sometimes referred to as a secondary containment. The annular space is used to detect any leakage that may occur in the inner or the outer pipe, in addition to enhancing safety against leakage onto the surrounding ground. To this end, the space may contain a gas, suitably air that has a pressure in excess of or below the ambient pressure. The occurrence of a leakage can be detected, by monitoring the pressure in said space. It is important that pressure changes in the space between the pipes are able to propagate along the full length of a monitoring zone in a conduit system or pipework. When joining such pipes, the inner pipe must be joined per se and the outer pipe must be joined per se, such that communication is also obtained between the annular spaces in the mutually joined double conduits.
EP 1 062 454 A discloses a previously known electro fusion coupler for joining together double walled conduits. This known coupler has a larger diameter at its ends than along an intermediate part thereof. The diameter at said ends is adapted to receive an outer pipe with a close fit and the diameter along said intermediate part is adapted to receive with a close fit an inner pipe of a double-walled conduit that includes two coaxial pipes and a leakage detection space between said pipes. The socket includes heating coils of resistance wires or filaments that when energized function to locally fuse the socket material with the material of both the outer pipes and the inner pipes in two double-pipe conduits to be joined together with the aid of the fusion welding socket. The fusion welding socket is further provided with a number of passageways arranged in the intermediate part for connecting together the leakage detection spaces of the two conduits. These passageways are formed as longitudinal cylindrical through holes drilled in the intermediate part or as two to four narrow tubular elements, such as steel tubes that arranged longitudinally in the intermediate part by being moulded into the socket during manufacturing.